The present invention relates to a method of preparing a hydrophobic partially aggregated colloidal silica and more particularly to a method comprising reacting a silca sol comprising at least one hydrophilic partially aggregated colloidal silica with an organosilicon compound. The present invention also relates to the hydrophobic partially aggregated colloidal silica prepared by the method.
Silica fillers are widely used in the rubber industry to reinforce various silicone and organic elastomers. These fillers improve the mechanical properties, for example, hardness, tensile strength, elongation, modulus, and tear resistance, of the vulcanized rubber. However, finely divided silica fillers, such as colloidal silica, tend to agglomerate and are hard to disperse in uncured elastomer compositions. Consequently, silica fillers are often treated with an organosilicon compound to render the surface hydrophobic. Treatment reduces the tendency of the filler particles to agglomerate and also prevents premature crepe hardening.
Methods of preparing hydrophobic non-aggregated colloidal silica are known in the art. For example, Castaing et al. disclose a method of prepairing nanosized hairy grains by grafting long polydimethylsiloxane chains onto silica particles having a radius of about 15 nm (Europhys. Lett., 1996, 36 (2), 153-158).
Kwan et al. disclose the synthesis of colloidal silica by silylation of an aqueous suspension of colloidal silica with either chlorosilanes or disiloxanes in the presence of acid and isopropyl alcohol, without aggregation of the silica particles (156th ACS Rubber Division Meeting, Orlando, Fla., September 1999, Paper 96).
U.S. Pat. No. 6,025,455 to Yoshitake et al. discloses a process of producing a hydrophobic organosilica sol which comprises aging a reaction mixture at 0 to 100xc2x0 C. in the state that an alkali present in the reaction mixture is removed or is neutralized with an acid in an equivalent amount or more, thereby forming a silylation-treated silica sol having a hydrophobic colloidal silica dispersed therein, the reaction mixture comprising: (a) a hydrophilic colloidal silica having a specific surface area of 5.5 to 550 m2/g in SiO2 concentration of 5 to 55% by weight, (b) a silylating agent of a disiloxane compound and/or monoalkoxysilane compound in a millimolar ratio of 0.03 to 2 in terms of Si atom per 100 m2 of a surface of the hydrophilic colloidal silica, and (c) a medium, as a residue thereof, comprising a mixed solvent comprising a hydrophobic organic solvent having a solubility of water of 0.1 to 12% by weight and an alcohol having 1 to 3 carbon atoms in a weight ratio of 0.05 to 20 to the hydrophobic organic solvent, and water in an amount of 15% by weight or less based on the weight of the medium.
U.S. Pat. No. 6,051,672 to Burns et al. discloses a method for making hydrophobic non-aggregated colloidal silica comprising reacting an aqueous suspension of a hydrophilic non-aggregated colloidal silica having an average particle diameter greater than about 4 nm with a silicon compound at a pH less than about 4 in the presence of a sufficient quantity of a water-miscible organic solvent to facilitate contact of the hydrophilic non-aggregated colloidal silica with the silicon compound at a temperature within a range of about 20 to 250xc2x0 C. for a time period sufficient to form a hydrophobic non-aggregated colloidal silica.
Although the aforementioned references describe various methods of producing hydrophobic non-aggregated colloidal silica, they do not teach the method or the hydrophobic partially aggregated colloidal silica of the present invention.
The present invention is directed to a method of preparing a hydrophobic partially aggregated colloidal silica, the method comprising reacting (a) a silica sol comprising at least one hydrophilic partially aggregated colloidal silica with (b) an organosilicon compound selected from (i) at least one organosilane having the formula R1aHbSiX4xe2x88x92axe2x88x92b, (ii) at least one organocyclosiloxane having the formula (R12SiO)m, (iii) at least one organosiloxane having the formula R13SiO(R1SiO)nSiR13, and (iv) a mixture comprising at least two of (i) (ii), and (iii), in the presence of (c) water (d) an effective amount of a water-miscible organic solvent, and (e) an acid catalyst, to produce the hydrophobic partially aggregated colloidal silica and an aqueous phase, wherein R1 is hydrocarbyl or substituted hydrocarbyl; X is a hydrolysable group; a is 0, 1, 2, or 3; b is 0 or 1; a+b=1, 2, or 3, provided when b=1, a+b=2 or 3; m has an average value of from 3 to 10; and n has an average value of from 0 to 10.
The present invention is also directed to the hydrophobic partially aggregated colloidal silica produced by the aforementioned method.
The method of the present invention produces hydrophobic partially aggregated colloidal silica in high yield from commercially available starting materials. Moreover, the method can be conveniently controlled to produce hydrophobic silica having a wide range of hydrophobicity, surface area, pore diameter, and pore volume.
The hydrophobic partially aggregated colloidal silica improves the mechanical properties of silicone and organic elastomers (i.e., cross-linked polymers). In particular, a low concentration of the silica imparts excellent mechanical properties, such as durometer hardness, tensile strength, elongation, modulus, and tear strength, to a silicone elastomer compared with hydrophobic non-aggregated colloidal silica. The potential advantages of low silica concentrations include shorter formulation time, lower cost, and lower viscosity.
The hydrophobic silica of the present invention can be used as a reinforcing or extending filler in organic polymer compositions, such as epoxy resins and phenolic resins, and silicone compositions, such as sealants, adhesives, encapsulants, and conformal coatings.